Leukemic chickens are being studied for the reexpression of fetal gene products. The objectives are to (1) determine if the reexpression of fetal gene products on adult leukemic chicken red blood cells is due to the derepression of fetal genes or is due to the release of immature red blood cells which naturally express these fetal gene products at their particular stage of maturation, and to (2) determine whether fetal genes are expressed as families of genes. Fetal markers include four red blood cell markers, two hemoglobin markers, a serum protein marker, and a B-cell Fc receptor. These established markers will be qualitatively and quantitatively examined in normal adult chicken bone marrow, spleen, liver, and kidney. Efforts will be made to identify cell types expressing specific gene products. Two virally induced leukemias (myeloid and erythroid) will be utilized to determine the number of fetal gene products that are expressed in these leukemic states. Cell to cell hybridization studies will be performed in order to determine if fetal gene products can be induced to reappear in cells from adult chickens. BIBLIOGRAPHIC REFERENCES: Ewald, S., L. Freedman and B.G. Sanders, 1976. EA rosette forming lymphoid cells: Distribution as to cell types and organs from chickens of different developmental stages. Cell. Immunol. 23(1): 158-170. Ewald, S., L. Feedman and B.G. Sanders, 1976. EA rosette-forming lymphoid cells in chickens: Specificity of the Fc receptor and its relationship to other surface antigens. Immunol. 33 (In press).